1. Field of the Invention
The present invention relates to a method and a device for measuring the thickness of a vertically aligned liquid crystal (hereinafter referred to as xe2x80x9cVA liquid crystalxe2x80x9d) being sealed in a panel.
2. Description of Related Art
Liquid crystal display panels comprise glass layers at their top and bottom with the intermediate layer being filled with a liquid crystal. The portion to be filled with the liquid crystal is previously filled with air until the liquid crystal is sealed therein.
There has been a conventional method of measuring the thickness (cell gap) of a liquid crystal sealed in a panel, in which light is directed from above the panel without the liquid crystal sealed therein so that reflection interference of the light is measured to obtain a thickness of the air layer, which is regarded as the thickness of the liquid crystal (Reflection-interference method).
However, the thickness of the air layer and that of the liquid crystal after being sealed in are not identical in a strict sense. Accordingly, developing a method for directly measuring the thickness of a liquid crystal after it is sealed in has been awaited.
Another known method is a method in which the birefringence property of uniaxial crystals that liquid crystals have is utilized in such a manner that light is directed through a polarizer perpendicularly to the surface of a liquid crystal panel with a liquid crystal sealed therein so as to measure an optical transmittance. Based on the optical transmittance, a birefringent phase shift (which is called xe2x80x9cretardationxe2x80x9d) of the liquid crystal is determined, from which the cell gap is obtained. (Refer to H. L. Ong, Appl. Phys. Lett. 51 (18), Nov. 2, 1987, ppl398-1400, Japanese Unexamined Patent Publication Nos. H4-307312 and H2-118406.)
Although the above-mentioned method is an excellent measuring method utilizing the birefringence property of liquid crystals, it is only effective for liquid crystals whose crystal axis is in a direction parallel to the panel surface, and inapplicable to liquid crystals including the VA liquid crystal whose crystal axis is in a direction perpendicular to the panel surface. This is because, as far as uniaxial crystals are concerned, the xe2x80x9cliquid crystal whose crystal axis is in a direction perpendicular to the panel surfacexe2x80x9d is not different from an isotropic liquid crystal when viewed from the light traveling direction.
Although the reflection-interference method mentioned above may be employed for the isotropic liquid crystal, due to proximity of the refractive index of liquid crystal to that of glass, only weak interfering light is expected to generate, which causes a great margin of error in the measured film thickness.
It is therefore an object of the present invention to provide a method and device for accurately measuring the thickness (cell gap ) of a VA liquid crystal having a birefringence property whose optical axis is in a direction perpendicular to the surface of the liquid crystal panel.
The method of measuring a cell gap of a VA liquid crystal according to this invention comprises the steps of: extracting a fixed polarized component from light of a light source; directing the light of the polarized component to the liquid crystal panel such that the light is incident obliquely on an optical axis of the VA liquid crystal panel; measuring transmittance intensity of a specific, polarized component of the light that has passed through the VA liquid crystal panel; determining a retardation R of the VA liquid crystal based on the transmittance intensity; and obtaining a thickness of the VA liquid crystal by means of the retardation R and data of an ordinary refractive index no and an extraordinary refractive index ne of the VA liquid crystal.
The xe2x80x9clight of the light sourcexe2x80x9d mentioned above may be a monochromatic light or a polychromatic light (e.g. white light). In the case of a monochromatic light, the measurement of the transmittance intensity can be carried out directly by a light receiving element. On the other hand, in the case of a polychromatic light, the measurement of the transmittance intensity is carried out by the light receiving element via a spectroscope.
In this method, by directing the light such that the light is incident obliquely on the optical axis of the liquid crystal panel, birefringence caused only by the liquid crystal layer is artificially generated. Accordingly, the thickness of the VA liquid crystal can be accurately measured.
The device for measuring a cell gap of a VA liquid crystal according to this invention comprises: a light source; a polarizer for extracting a fixed, polarized component from the light of the light source; optical axis setting means capable of directing the light of the polarizer to the VA liquid crystal panel such that the light is incident obliquely on an optical axis of the VA liquid crystal panel; an analyzer for extracting a specific, polarized component of the light that has passed through the VA liquid crystal panel; a light receiver for measuring transmittance intensity of the analyzer; and data processor for obtaining a retardation R of the VA liquid crystal panel based on the transmittance intensity so as to obtain a thickness of the VA liquid crystal by means of the retardation R and data of an ordinary refractive index no and an extraordinary refractive index ne of the VA liquid crystal.
This device includes optical axis setting means capable of directing the light to the VA liquid crystal panel such that the light is incident obliquely on the optical axis of the VA liquid crystal panel. Directing the light so as to be incident obliquely on the optical axis of the VA liquid crystal allows birefringence that results only from the liquid crystal layer to be artificially generated. Accordingly, the thickness of the VA liquid crystal panel can be measured accurately.